This invention relates to composite shim structures. More particularly, this invention relates to composite shim structures for use in high capacity laminated bearings.
It is generally known that the compressive load carrying capacity of resilient material may be increased by forming a lamination that includes alternating layers of resilient material and non-extensible material where the layers are oriented generally perpendicular to the direction of the anticipated compressive load. Such laminations have been used as bearings. One such bearing, commonly known as a high capacity laminated (HCL) bearing, has found commercial acceptance for a variety of applications including use as helicopter rotor bearings.
The commercial acceptance of HCL bearings may be due to the unique characteristics provided by their laminate structure. The laminate structure may increase the compressive load carrying capacity of the resilient material while retaining the ability of the resilient material to yield in shear or in torsion directions parallel to the laminations. As the expected compressive load increases, the rubber layers may have to be kept thin to reduce compression bulge strains. However, an overall thickness of rubber may be needed to accommodate torsional motion. Thus, a significant number of resilient layers may be needed to support a given load. Accordingly, a significant number of non-extensible layers, commonly known as shims, may also be required.
Shims should be capable of handling compressive loads on the bearing as well as supporting stresses in the radial and circumferential directions. Shims are typically thin metal plates, formed of brass, aluminum, titanium, steel or stainless steel. While metallic shims may provide the desired physical characteristics for many applications, these shims may increase the bearing cost and add a significant amount of unwanted weight to the bearing structure, particularly when a large number of layers are required to obtain the desired torsional characteristics. In some instances metal shims do not have sufficient strength or stiffness to endure the continuous loading that is experienced in some applications.
Composite or reinforced plastic shims may also be used in place of metal shims or in cases where metal shims are not adequate. Examples of such composite shims are taught in U.S. Pat. No. 4,108,508 to Clinard, Jr., which is assigned to the present assignee, as well as in U.S. Pat. Nos. 4,263,243 to Wilson et al., 4,708,758 to McGregor, and 5,297,874 to Raines. While composite shims may provide the desired reduction in bearing weight and cost, these shims may not provide other desired physical characteristics such as high strength and high stiffness.
The foregoing illustrates limitations known to exist in present devices and methods. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, suitable alternatives are provided including features more fully disclosed hereinafter.
It is, therefore, an object of the present invention to provide a shim that may combine the desired physical characteristics of a metallic shim with the weight and cost characteristics of a composite shim.
It is another object of the present invention to provide a shim that has a greater strength and stiffness than a metal shim.
It is another object of the present invention to provide bearing configurations utilizing such shims.
These and other objects may be provided in one aspect of the invention by providing a composite shim having a laminate structure that includes a first composite layer having at least one fiber that substantially surrounds a portion of the layer, such as the center of the first composite layer for example. The composite shim may include more than one first composite layer. By employing such laminate structures, the present invention may provide shims that possess better strength and stiffness characteristics than those possessed by metallic shims as well as the desirable weight and cost characteristics generally associated with composite shims.
According to one embodiment of the present invention, the at least one fiber of the first composite layer is comprised of a fiber that forms a spiral pattern. According to another embodiment of the invention, the at least one fiber of the first composite layer is comprised of a plurality of concentric circumferentially extending circles. According to other embodiments of the present invention the first composite layer may also be comprised of a combination of at least one radially oriented fiber and at least one fiber oriented circumferentially.
According to another embodiment of the present invention, the laminate structure of the composite layers comprises at least one second composite layer that includes at least one fiber that is substantially radially oriented.
According to another embodiment of the present invention, the laminate structure comprises at least one third composite layer that includes at least one fiber oriented uniaxially. The laminate structure of the invention may include a plurality of third composite layers each having the respective at least one fiber oriented along an axis with the third composite layers of the laminate structure arranged so that the axes of consecutive third composite layers are offset by an angle. The consecutive third composite layers may be adjacent or may be separated by a first or second composite layer. The axes of the fibers may be arranged so that the layers are oriented at either 0xc2x0, +45xc2x0, xe2x88x9245xc2x0 or 90xc2x0.
According to another aspect of the present invention, the composite shim is comprised of a laminate structure that includes a combination of the first, second, and third composite layers.
According to another embodiment of the present invention, a laminated bearing structure includes a plurality of resilient layers and a plurality of shims alternating with and laminated to the plurality of resilient layers. At least one of the shims is a composite shim having a laminate structure. The laminate structure includes a first composite layer having at least one fiber that substantially surrounds a predetermined location along the first composite layer.
According to another embodiment of the present invention the laminated structure of the present invention is tubular and includes a plurality of fibers oriented circumferentially around an axis and a plurality of fibers oriented substantially parallel to the axis.
The foregoing and other aspects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.